This application claims the benefit of Korean Patent Application No. 2001-41251, filed on Jul. 10, 2001, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to a circuit that protects thin film transistors (TFTs) of a liquid crystal display (LCD) device from electrostatic discharge.
2. Discussion of the Related Art
Cathode-ray tubes (CRTs) have been mainstream display devices for many applications. However, various flat panel display devices that are smaller, lighter, and consume less power have been developed. In particular, the thin film transistor liquid crystal display (TFT-LCD), which is very thin and possess excellent color characteristics, has been highly developed and has become commonplace.
Generally, a liquid crystal display device is a device for displaying images corresponding to data signals that are individually applied to pixels that are aligned in a matrix. The pixels control light transmittance to produce an image. Thus, a liquid crystal display device includes both a pixel matrix and driver integrated circuits (IC) for driving the pixels.
FIG. 1 is a cross-sectional view showing a partial cut-away view of a TFT-LCD display, and FIG. 2 is a schematic circuit diagram showing a TFT-LCD. Hereinafter, the respective components will be described with reference to the drawings.
In the TFT-Array display, a TFT substrate (the lower substrate in FIG. 1) is formed with two or more metallic layers, an insulating layer, an amorphous silicone layer, an indium-tin-oxide (ITO) layer and other required elements are deposited on a glass substrate 102 to form a TFT 107, a storage capacitor 108, a pixel electrode 104, and other structures to form an individual pixel. In addition, the TFT substrate includes data lines that interconnect multiple pixels to form a pixel matrix. Additionally, bonding pads 106 at the ends of respective data lines are used to applying data signals.
FIG. 1 also shows a color filter substrate (the upper substrate in FIG. 1) formed on a glass substrate 101. The color filter substrate includes a black matrix 109 (beneficially formed of Cr) that selectively blocks light and RGB color filters 110 over respective pixels of the TFT substrate. Additionally, an ITO thin film 103, which forms a common electrode, is deposited across the bottom of the color filter substrate.
On the substrates are alignment films 111 for aligning liquid crystal molecules in predetermined directions. The TFT and color filter substrates form a gap that is maintained uniformly by spacers 112. Liquid crystal is disposed in the gap.
An electrical connection is formed between a voltage applying terminal of the TFT substrate and the ITO thin film 103 by silver dots 114. This enables voltage to be applied to the common electrode (the ITO thin film 103).
A patterned seal 113 positioned around the circumference of the substrates functions as an adhesive that fixes the TFT-Array substrate and the color filter substrate together. The seal 113 also maintains liquid crystal between the two substrates.
Referring now to FIG. 2, on the TFT substrate 102 are a plurality of data lines for transmitting data signals applied from a data driver integrated circuit 201 to the pixels, and a plurality of gate lines for transmitting gate signals applied from a gate driver integrated circuit 202 to the pixels. The data and gate lines are formed orthogonally. Bonding pads 106 (see FIG. 1) to which the data signals and the gate signals are applied are formed at end portions of the data and gate lines. The individual pixels are positioned near the crossings of the data and gate lines.
The gate driver integrated circuit 202 applies gate signals to the plurality of gate lines such that the pixels are selected line by line, while the data signals are applied to the pixels in the selected line.
The TFTs 107 (see FIG. 1) are used as switching devices and are formed in the individual pixels. When a gate signal is applied to the gate electrode of a TFT via a gate line, a conductive channel is formed between the source and drain electrodes of the TFT. Then, an applied data signal, which is applied to the TFT drain electrode via a data line, controls the light transmittance of that pixel.
Since the glass substrates 101 and 102 are insulators, static electricity generated during the fabrication process of the TFT-Array can collect on the glass. Also, static electricity can be generated by various treatments applied to the various substrates. Such static electricity can result in electrostatic-discharge damage to the TFT-Array. Furthermore, static electricity can cause dust particles to be attracted to the glass substrate, which can contaminate the TFT-Array and the color filter array.
To reduce static electricity, the fabrication equipments and the various process used to produce a TFT LCD can be treated to minimize static electricity. However, a well-designed TFT-Array still must incorporate protection against electrostatic-discharge.
Static electricity is a particular problem because the TFT devices used in the TFT-Array are prone to static damage because the gate insulating film can be easily destroyed by relatively energy levels. Therefore, to protect the TFT-Array the induction of static electricity in the gate and data lines must be prevented. One way of doing this is to electrically short the gate signal lines and the data signal lines together. For example, if static electricity is generated between a gate line and an adjacent data line, by making the 2 lines have an equipotential damage can be prevented.
While directly connecting the gate and data lines together is efficient, such direct connections prevent electrical testing to determine breaks in signal lines or defective TFTs. Furthermore, operational tests cannot be performed. Therefore, a protection circuit that protects against electrostatic-discharge damage but enables examination of the individual pixels has been developed. That protection circuit is comprised of elements located between respective gate lines and a gate shorting line, and between respective data lines and a data shorting line. The protection circuit is illustrated in FIG. 3.
FIG. 3 shows a plurality of gate lines (G1 through G768) that are formed on a substrate 102 in a row direction. FIG. 3 also shows a plurality of data lines (D1 through D3072) that are formed on the substrate 102 in a column direction. Also shown is a gate shorting line GSL, a data shorting line DSL, and the ITO layer that forms the common electrode. The gate shorting line receives a gate low level voltage (Vgl) while the data shorting line DSL receives a common voltage (Vcom).
FIG. 3 also shows a plurality of gate line ESD protection units, GESD1 through GESD768 and GLESD1 through GLESD768, and a plurality of data line ESD protection units, DESD1 through DESD3072 and DLESD1 through DLESD3072. The gate line ESD protection units connect the front ends of the gate lines G1 through G768 to the gate shorting line GSL, while the data line ESD protection units connect the front ends of the data lines D1 through D3072 to the data shorting line DSL. Additionally, ESD protection connection units CESD1 and CESD2 connect the gate shorting line GSL to the data shorting line DSL. Finally, ESD protection induction units IESD1 and IESD2 connect the data line ESD protection units DESD1 and DESD3072 to the ITO.
When an image is being produced, a low level voltage Vgl is applied to all of the gate lines, except the gate line that is currently being driven. That driven line receives a high gate voltage that turns on the TFTs connected to that line. Thus, the gate line voltage is either Vgl or a high gate voltage. Because the protection circuit shown in FIG. 3 protects against high (static) voltages it is beneficial to connect the gate shorting line GSL to the gate low level voltage Vgl. That way, the protection devices (such as GESD1 through GESD768) are stressed by the difference between the high gate voltage and Vgl (rather then the difference between the high gate voltage and ground). If an abnormal signal, such as noise, or low level static electricity, is produced across the ESD protection unit, the ESD protection unit can conduct and impact on adjacent gate lines. Otherwise, with the gate voltage Vgl applied to the gate shorting line GSL there is no voltage across the ESD protection unit (except the one that receives the high level voltage). This stabilizes the state of the ESD protection units. Also, by applying Vcom to the data shorting line DSL the ESD protection units connected to the DSL are stabilized.
Hereinafter, the operation of the ESD protection circuit shown in FIG. 3 will be described. First, when high voltage static electricity is produced in one of the gate lines G1 through G768, the associated gate line ESD protection units GESD1 through GESD768, which are attached on the front ends of the gate lines G1 through G768, are turned on, thus dispersing static electricity to all of the gate lines by way of the gate shorting line GSL and the other ESD protection units (which act bi-directionally). Additionally, the gate line ESD protection units GLESD1 through GLESD768, which are connected to the rear ends of the gate lines G1 through G768, are turned on, thus dispersing static electricity to the gate shorting line GSL (and thus to the other gate lines). Additionally, static electricity is dispersed to the data shorting line DSL by the connection ESD protection units CESD1 and CESD2.
Charges that pass through the connection ESD protection units CESD1 and CESD2 are then dispersed via the data shorting line DSL through the induction ESD protection units IESD1 to the data lines D1xcx9cD3072 via data protection units DESD1 through DESD3072. Additionally, charges on the data shorting line DSL are dispersed to the data lines D1xcx9cD3072 through the data line ESD protection units DLESD1 through DLESD3072. The above process thus disperses charges on a gate line to all of the gate and data lines. Furthermore, since all of the protection units are bi-directional, charges on a data line are dispersed to all of the gate and data lines.
While the protection scheme shown in FIG. 3 has proven useful, problems have been found. For example, the induction ESD protection unit IESD1 essentially connects the data line ESD protection units DESD1 through DESD3072 to the connection ESD protection unit CESD1. The induction ESD protection unit IESD1 has an internal resistance. Furthermore, it has been found by experimentation that, in practice, most of the static electricity generated charges that pass through the connection ESD protection unit CESD1 are dispersed into the upper substrate (the ITO) via the Ag dot 114. This is believed to be because the upper substrate represents a lower resistance to static charges than the induction ESD protection unit IESD1. Thus, the dispersion of static electricity is less than optimal, which increases that chance of damage caused by static electricity.
Therefore, an improved ESD protection circuit would be beneficial. In particular, an improved ESD protection circuit that better distributes static electric charges into and from the data lines would be beneficial.
Therefore, an objective of the present invention is to provide an electrostatic discharge protection circuit for a TFT-LCD that more efficiently dispersing static electricity to and from gate lines and data lines.
To achieve advantages in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided an electrostatic discharge protection circuit for an LCD having a plurality of crossing gate and data lines on a substrate. The substrate further includes a data shorting line (DSL) and a gate shorting line (GSL). The substrate further includes a plurality of first ESD protection units for connecting the gate shorting line to the gate lines, a plurality of second ESD protection units for connecting the data shorting line to the data lines, third ESD protection units for connecting the gate shorting line to the data shorting line, and a common electrode on a second substrate. The protection circuit further includes a fourth ESD protection unit that directly connects the common electrode to the third ESD protection units.
The foregoing and other objectives, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.